1. Field of the Invention
This invention relates to a control system for an internal combustion engine with supercharger, specifically to a fuel control system for the engine in connection with a supercharging pressure for intake gas.
2. Description of the Prior Art
In the field of internal combustion engine for automotive, recently, a supercharger has been commonly used for supercharging intake gas for the engine to improve an output performance. As a supercharging pressure is increased, a charging effect of the intake gas is increased. Therefore, an increase of the supercharging pressure is desirable for increasing an engine output at one aspect. However, from a different aspect, this phenomenon is undesirable. This is because an undue increase of the supercharging pressure or over-supercharging of the intake gas causes an improper combustion, deteriorates a durability of the engine and affects a reliability of the engine operation.
In view of the above, there have been known various supercharging pressure control devices for restricting the supercharging pressure from an undue increase. For instance, a relief valve for an exhaust system such as a waste gate valve is employed for bypassing a part of exhaust gas from the supercharger to restrict the supercharging pressure below a predetermined value.
A relief valve for intake system such as an intake gas bypass valve is employed for bypassing a part of intake gas from the supercharger to suppress an undue increase of the supercharging pressure.
It should however be noted that the conventional supercharging pressure control devices are not actuated until the supercharging pressure has been unduly increased and therefore not responsive. Besides, when the control device is out of order or in an abnormal condition, it is difficult to control the supercharging pressure. Thus, the above supercharging devices are not sufficient to obtain a reliability of the engine operation.
Under the circumstances, Japanese Patent Public Disclosure No. 57-157016 laid open to the public on Sept. 28, 1982 has proposed a supercharging control system provided with a fuel control system for staying a fuel supply to the engine in an over-supercharged condition in addition to a supercharging pressure control device as aforementioned.
In this system disclosed in the Japanese Patent application, the supercharging pressure control device is actuated to restrict the supercharging pressure when the supercharging pressure is increased beyond a first predetermined value provided in accordance with an engine operating zone. In this case, a plurality of the engine operating zones are provided in accordance with an engine operating condition defined by an engine load, engine speed and the like. Thus, the first predetermined value changes depending on the engine operating condition as shown by a line A in FIG. 4 wherein the first predetermined value is increased as an engine output torque is increased. The fuel control system of the Japanese Patent application is actuated to stay a fuel supply when the supercharging pressure is increased beyond a second predetermined value which is greater than the first predetermined value and constant irrespective of a change of the engine operating zone as shown by a line C in FIG. 4.
It should be however noted that this control system as disclosed in the Japanese application is disadvantageous in the following points.
In the case where the supercharging pressure control device is out of control such as out of order, the fuel control is not actuated until the supercharging pressure is increased beyond the second predetermined value even when the engine operating condition is in an engine operating zone in which the first predetermined value is relatively small. Consequently, the fuel control is unlikely to be initiated actually because a difference between the first and second predetermined supercharging pressure is fairly big. This deteriorates a durability of the engine.